Field of Invention
The present invention relates to a technical field of orbital angular momentum (OAM) wireless communication, and more particularly to a traveling-wave loop antenna based on a metal ring cavity for generating a radio frequency OAM.
Description of Related Arts
With the global entry into the mobile internet age, the frequency spectrum deficiency in the mobile communication services is increasingly serious. Because the high-quality frequency spectrum resource in the low frequency band is limited, it is difficult to satisfy the demands of the mobile communication merely by dividing the frequency spectrums. Under the circumstance, it is particularly important to develop a technology to increase the frequency spectrum efficiency. Currently, people have made a great number of researches on expanding the information capacity resource based on the frequency spectrum, phase and amplitude of the electromagnetic wave, such as the cognitive radio which is an intelligent wireless communication technology for increasing the frequency spectrum utilization, the high-order coherent signal modulation for increasing the frequency spectrum efficiency of a single carrier, the multi-carrier technology which doubles the frequency spectrum efficiency as compared with the serial system, and the multiple-input multiple-output (MIMO) communication technology for increasing the frequency spectrum efficiency and multiplying the channel capacity. Obviously, the capacity resource based on the freedoms of electromagnetic wave, such as frequency spectrum, phase and amplitude, has been relatively fully developed and utilized. Although a further gradual capacity expansion based on these freedoms is feasible, no enough space exists for expanding the capacity by several orders of magnitude. Thus, it is a great scientific and technical challenge to provide another physical parameter as a new freedom to realize the electromagnetic wave communication technology and satisfy the increasing demands of the communication capacity of several orders of magnitude within the limited frequency spectrum resource. The OAM wireless communication emerges as a response to the challenge.
The electromagnetic wave carries OAM as well as energy. The OAM is a basic physical property of the electromagnetic wave and describes the azimuthal phase distribution of the electromagnetic wave around the propagation direction axis. For an electromagnetic wave at an arbitrary frequency, all the OAM beams constitute a group of eigen modes which are mutually orthogonal to each other and have an infinite number. The OAM communication adopts the OAM order (valued l), which is the eigen modes of the electromagnetic wave, to serve as an unexploited freedom for modulation and multiplexing. In other words, different values of OAM mode l are encoded by different information and represent different communication channels, hence OAM has the ability to further increase the frequency spectrum efficiency. Because of the unbounded range of l, the OAM communication has the potential to infinitely increase the capacity of the information carried by the electromagnetic wave theoretically.
Conventionally, the application of using radio frequency OAM as an unexploited freedom in the wireless communication field is still in the primary stage. Most of the researches thereof are focused on the theoretical analysis. The research and development, on generation and multiplexing of the radio frequency OAM beams and the related devices, is the basis for verifying the free space channel characteristics of the OAM beams and realizing the radio frequency OAM wireless communication system. So far, most of the OAM beam generation methods are originated from the conventional circular antenna array designed by Thide et al. in 2007. However, the order of the OAM beam generated by the conventional method of Thide et al. is limited by the number of the circular loop antenna array. Given that the number of the loop antenna array is N, the order l of the generated OAM beam must be smaller than N/2. Moreover, the conventional method of Thide et al. is disadvantageous for the multiplexing of the OAM beams. Thus, a simple and feasible conversion device based on the mature waveguide technology, which can convert the radio frequency guided-wave mode into the radio frequency OAM mode, shows a great practical significance for accelerating and facilitating the future radio frequency OAM high-speed communication.